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The recovery and purification of metals from metal-bearing ores has been practiced for millennia whereby metal containing substances, including native metals and metal salts, metal sulphides, metal oxides and various other forms are subjected to various recovery procedures to produce high purity metals or alloys. Non-metallic elements also have been recovered and purified by various other processes, some of which rely on recovery of the elements from a liquid state, as exemplified by the recovery of salts from brines, or from the gaseous state, as exemplified by the recovery and purification of gases from air.
Throughout history and continuing to the present, the selection of a particular ore for the recovery of elements of value is normally first made on the basis of visual indications of the presence of the element of interest or the presence of alteration minerals known to be associated with it. For example, although visible gold is not normally seen in gold deposits, minerals such as quartz, arsenopyrite and arsenian pyrite commonly are associated with gold, and their presence in a rock alerts the geologist to the possibility that the rock may contain gold in significant quantities. Similarly, exploration geologists and prospectors searching for copper will be alerted to the presence of a potential copper deposit by the bright blue and green colours of the minerals azurite and malachite which are the weathering products of copper mineral oxidation.
Accordingly, a useful prerequisite to recovering an element from a particular source material is a visual indication that the source material is enriched in the element sought. For base metals such as copper, and other substances of relatively lower value, visual estimates of the concentration of the element sought in the source material usually correspond well with the amount that is ultimately recovered. However, gold and precious metals and other elements of relatively higher value can occur in economically significant concentrations without visual indications of their presence or their enrichment.
It is desirable that accurate methods for determining the concentrations of gold and precious metals and other high-value elements are available when developing procedures for the recovery of these materials. Fire assay is an example of such a technique. Fire assays have been used for thousands of years to determine the concentrations of gold, silver and other precious metals in ores, rocks and concentrates. Fire assay works by segregating and concentrating the precious metals contained within the source material into a small bead from which the concentrations of the precious metals can be determined, either by weighing or, as is common today, by dissolving the bead in acid(s) and measuring its elemental composition by instrumental analysis.
Other methods have been used to determine the concentrations of metals and other elements in natural materials. The optical microscope and the more recently developed scanning electron microscope (SEM) and electron microprobe (EMP) have extended the range of visual estimation of element concentrations to the micron to sub-micron scale. Other instruments such as the transmission electron microscope (TEM) have extended the range of visual determination of element abundance to the scale of the atom. Various analytical techniques such as energy dispersive x-ray analysis can be used during electron imaging to determine the chemical composition of the substrate under the electron beam. These tools extend the ability of the geochemist and metallurgist to correlate between visual and chemical estimates of element concentrations from the macroscopic scale to the atomic scale. As such, they serve as a complementary method to chemical analysis for estimating the concentrations of small quantities of elements of high value.
These varying methods of determining useful content of metals have from time to time given differing results. That is, assessing a particular ore by the fire assay method may not show any significant concentration of the desired end product, whereas analysis using the SEM or EMP techniques may show the presence of such metals or products.
SEM and EMP examination of sedimentary rocks from selected areas in western Canada by a number of competent agencies has documented previously unknown occurrences of micron to submicron sized particles of native metals and intermetallic alloys. Amongst the metals identified by electron imaging as occurring in this form are the base metals chromium, manganese, iron, nickel, copper and zinc, and the precious metals including gold, silver and the platinum group metals (PGM). They are accompanied by a wide variety of other metallic and non-metallic elements. In all, some 56 elements of the periodic table have been identified as occurring in this form in these rocks. These deposits have been called xe2x80x9cPrairie-typexe2x80x9d deposits. Fire assay of these rocks for gold, platinum and palladium and other precious metals, however, typically returns values at or below their respective lower limits of detection. Therefore, the concentrations of precious metals in these rocks can not be accurately determined by conventionally practiced analytical techniques.
Allusion to difficulties in the detection and recovery of precious metals from ores containing precious and other metals has been set out in the literature and various attempts have been made to recover such metals from these ores by a variety of processes. For example, in Hunter, U.S. Pat. Nos. 3,150,960 and 3,238,038, there are disclosed processes for the recovery of platinum, gold, silver, palladium, ruthenium, iridium, rhodium and osmium from bituminous shales which conventional fire assay procedures frequently showed to be barren of these metals. Hunter""s work, directed particularly toward the recovery of gold and PGM""s, postulated that the precious metals, which occur in the xe2x80x9cshalesxe2x80x9d, had a tendency to resist xe2x80x9call heretofore known procedures for recovering them economically,xe2x80x9d were in a colloidal form or were xe2x80x9centangledxe2x80x9d with silica particles, and thus were not accessible to the action of conventional agents of recovery.
Similarly in Anderson, U.S. Pat. No. 3,958,985, there is disclosed an extraction method for non-ferrous metals for the recovery of precious metals and other non-ferrous metals from, xe2x80x9cso-called unassayable ores wherein the minerals are combined in such a way that they cannot be analyzed by conventional techniquesxe2x80x9d. Anderson disclosed that many ores contain both conventionally detectable and recoverable metals including precious metals and other precious and base metals that are not normally detected. That is, the then-current assay methods xe2x80x9cidentified only a portion of the metal present in the samplexe2x80x9d. Anderson gives no information as to the precise nature of the precious or other non-ferrous metals.
More recently Butler, U.S. Pat. No. 5,215,575, disclosed a process for processing noble metal-containing ores at low pulp densities where the noble metals are reported to be, xe2x80x9c . . . in extremely fine form and are often present in higher concentrations than is revealed by normal assay techniques in common usexe2x80x9d. In Butler, the presence of metal-absorbing substances, specifically clays, carbon or sulphides, is suspected to remove some of the metals from solution so that they cannot be detected by instrumental analysis. This mechanism is invoked to explain the loss of gold from cyanide leach recovery and aqua regia assay solutions, as well as loss of gold to slag in fire assay.
Although natural occurrence of nanoclusters has not been reported in the technical literature, manufactured nanoclusters are known and are becoming increasingly important in the fields of catalysis, ceramics, semiconductors, and materials science, among others. Their importance is due to the high ratio of surface atoms to interior atoms in nanoclusters. This imparts properties such as high surface reactivities, increased hardness and yield strength, decreased ductility, liquid-like behaviour at low temperature, and size-related quantum effects that are distinct from those properties of their macro-scale counterparts.
A method for production of nanoclusters at a high rate is disclosed in Brown, U.S. Pat. No. 5,958,329, wherein nanoclusters are produced by using an electron beam gun under high vacuum to vaporize source materials, which vapor is cooled in a condensation chamber to produce nanoclusters at claimed rates of up to kilograms per hour. Another method for artificially producing monodisperse sized nanoclusters is disclosed in Martino, U.S. Pat. No. 5,814,370.
At its finest division an element consists of a single atom. Molecules consist of simple aggregates of a few atoms, and metals and other macrocrystalline solids comprise a crystalline lattice extending outwards in continuous, three dimensional arrays of atoms. The dimensions of atoms and molecules are measured in angstroms ( ), one angstrom being 10-10 m or 0.1 nanometers (one nanometer, or nm, being 10-9 meter). Crystalline domains in macrocrystalline solids such as metals typically are measured on the scale of micrometers (micron or xcexcm) which is 10-6 meter. Nanoclusters occupy the transition from the simple atomic/molecular state to the crystalline state and have diameters in the range of 0.1 to about 100 nm. They consist of atomic polyhedra made up of as few as three or as many as thousands of atoms. Numerous terms have been used to describe nanoclusters. Smaller, charged nanoclusters are referred to as polyanions and polycations. Larger entities are referred to as microclusters, clusters or nanoparticles, although the more apt term nanocluster is now more commonly coming into use and has been adopted herein. Adjectives such as xe2x80x9cnanophasexe2x80x9d, xe2x80x9cnanocrystallinexe2x80x9d and xe2x80x9cnanoscalexe2x80x9d are also used to distinguish nanoclusters from their macroscale counterparts.
Within the context of the present invention, the following definitions are used:
xe2x80x9cAtomxe2x80x9d refers to the entity making up the smallest division of an element.
xe2x80x9cMoleculexe2x80x9d refers to entities comprising simple combinations of a few atoms with fixed chemical, physical and quantum properties.
xe2x80x9cNanoclusterxe2x80x9d refers to particles in the 0.1 to 100 nm size range which exhibit size-dependent variations in chemical, physical and quantum properties.
xe2x80x9cNanocluster gelxe2x80x9d refers to aggregates of dehydrated colloidal nanoclusters which commonly form micron to sub-micron sized particles.
xe2x80x9cMacrocrystalline solidsxe2x80x9d refers to crystalline substances, such as metals, that exhibit no size-dependent variation in chemical, physical or quantum properties, and whose crystal size is typically one micron or greater.
Throughout this disclosure and claims, the term xe2x80x9cmetalsxe2x80x9d is intended to include precious or noble metals comprising gold, silver and the platinum group metals (PGM) platinum, palladium, osmium, iridium, ruthenium and rhodium, group I alkali metals, group II alkaline earth metals, group III to group XII transition metals, and other metallic elements including aluminum, silicon, gallium, germanium, arsenic, indium, tin, antimony, tellurium, thallium, lead, bismuth, thorium and uranium, and the rare earth elements.
This invention relates to the field of metallurgy. In particular, the invention relates to the recovery of extremely small, naturally occurring particles known as nanoclusters, from host materials comprising solids, liquids and gases, and the resulting nanoclusters isolated thereby.
One object of the present invention is to provide a method for the detection and recovery of nanoclusters from nanocluster-bearing materials.
A further object of the present invention is to provide a method for the detection and recovery of metal nanoclusters from natural materials.
A further object of the present invention is to provide a method for recovery of metals from naturally occurring nanoclusters.
A further object of the present invention is to provide isolated natural nanoclusters of precious metals and/or PGM""s.
These and other objects of the present invention have been satisfied by the discovery that nanoclusters do, in fact, occur naturally. Heretofore these particles have not been observed and no attempt has been made to separate such naturally occurring nanoclusters for purposes of recovering metallurgically significant materials.
Accordingly, this invention further relates to a process for recovering naturally occurring nanoclusters from nanocluster-bearing source material in which the nanoclusters can be demonstrated to occur: i) as discrete nanoclusters which may be adsorbed to a host substrate, ii) as nanocluster colloids, or iii) as nanocluster gels, the process comprising forming an aqueous nanocluster slurry comprising the nanoclusters and host substrate, contacting the slurry with peptizing reagents to cause the formation of nanocluster colloids and isolating the nanocluster colloids.